The invention relates to an X-ray detector for converting electromagnetic radiation, notably X-rays, into electric charge carriers. The invention also relates to a method of operating an X-ray detector and to a method of manufacturing an X-ray detector. Furthermore, the invention also relates to an X-ray examination apparatus provided with an X-ray detector.
X-ray detectors are used notably in the medical field, that is, especially for X-ray examinations, and serve to form X-ray images of an object (normally a patient) to be examined in the context of usually a medical examination or therapy. An image acquisition system provided with an X-ray detector is used to form images of the object to be examined which is penetrated by the X-rays, said images being output, for example, via a monitor. The X-rays which are incident on the X-ray detector are converted into electric charge carriers in a converter arrangement. The electric charge carriers generated in the converter arrangement are collected in associated capacitances so as to be read out by a downstream electronic read-out circuit arrangement.
Generally speaking, an X-ray detector is constructed in such a manner that the electromagnetic X-rays are incident on a converter arrangement. Depending on the specific construction of the X-ray detector, either a directly converting converter layer in the converter arrangement converts the X-rays into electric charge carriers which are subsequently read out, or a converter arrangement which consists of two converter layers first converts the X-rays into visible light in a scintillator arrangement, after which the visible light is converted into electric charge carriers in a second converter layer which is arranged therebelow, notably a photosensor arrangement, said charge carriers subsequently being read out.
JP 5180945 describes an arrangement in which an infrared lamp applies heat during the manufacturing process of the scintillator layer. This lamp is not included in the X-ray detector but is mounted at a distance from and over the overall arrangement. A layer which is provided on the scintillator layer and absorbs infrared radiation generates heat upon irradiation so that the scintillator is heated. The application of heat is intended to remove irregularities in the scintillator or to homogenize the doping. To this end, the scintillator material is heated after its vapor deposition on a substrate.
Generally speaking, flat dynamic X-ray detectors include a scintillator arrangement of doped cesium iodide (CsI) which exhibits an increased sensitivity to X-rays when exposed to strong X-rays. The relative increase amounts to from 5 to 10% and decays only over a period of several days. This effect is referred to as bright burn, is spatially inhomogeneous and reaches its maximum value in the directly irradiated areas of the X-ray detector. This bright burn effect is very detrimental, because a different number of light quanta is generated from the same number of X-ray quanta in the areas in which such an increased sensitivity occurs, that is, in comparison with areas in which no increase of the sensitivity occurs. This gives rise to undesirable intensification and/or attenuation in the X-ray image.
Image artefacts are also liable to occur in directly converting X-ray detectors in which incident X-rays are converted directly into electric charge carriers by means of a lead oxide or selenium layer.
The bright burn effect cannot be corrected by image post-processing. It can be corrected only by further exposure for the formation of sensitivity correction images, that is, so-called gain images.
In typical circumstances such an increase of the sensitivity may not be noticeable in clinical images. The bright burn effect as caused by an unusual exposure or series of exposures or incorrect operation, however, can be cancelled neither deliberately nor in an accelerated manner.
Moreover, the calibration images used for the sensitivity correction must be updated at regular time intervals. It is by no means possible to wait for possibly present bright burns to decay for these calibration images, because in those circumstances no images can be formed for several days.
The gain images can additionally be used to monitor changes of the scintillator in the context of adaptive calibrations and a remote maintenance service. The order of magnitude of such changes may range from a few millimeters up to the size of the detector. The calibration and monitoring are impeded or even made impossible by the bright burn effect in the case of a spatially inhomogeneous relative increase by several percents.
Deep trap states are probably responsible for the long decay time of the bright burn effect.